Cyclic steam injection and gas drive



s- 2, 1969 E. R. GUM ET AL 3,460,621

CYCLIC STEAM INJECTION AND GAS DRIVE Filed May 22, 1967 2 Sheets-Sheet 1 STEAM OR AIR AIR OR GAS ROSCOE F VANDAVEER FIG 2 EARL R. GUM

, INVENTORS ATTORNEY Aug. 12,1969 E. R. GUM ETAL 3,460,621

CYGLIC STEAM INJECTION AND GAS DRIVE Filed May 22, 1967 2 Sheets-Sheet 2 ROSCOE F. VANDAVEER EARL R. GUM

INVENTORS AT TORNE Y 3,460,621 CYCLIC STEAM INJECTION AND GAS DRIVE Earl R. Gum and Roscoe F. Vandaveer, Tulsa, Okla., as-

signors to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed May 22, 1967, Ser. No. 640,043 Int. Cl. E21b 43/14, 43/22, 43/24 US. Cl. 166-263 4 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to the recovery of heavy oils and tars from underground deposits thereof. More particularly, it is concerned with a novel method for the recovery of such materials involving heating the latter by cyclic steam injection or the equivalent and then producing the oil or tar by means of a combination of gravity drainage and gas drive. As used in the present description, the term petroleum is intended to include tars and viscous oils as well as high gravity, e.g., 40 A.P.I. crudes.

Background of the invention In many areas of the world, large deposits of petroleum exist which, because of their relatively low gravity, either cannot be produced or can only be produced inefficiently by conventional methods. Such deposits include the Athabasca tar sands in Canada, low gravity crudes in the Iobo Field in Venezuela, and similar crudes in western Missouri, eastern Kansas and southern Oklahoma. Numerous proposals have been advanced for recovering petroleum of the type contemplated herein, some of which have involved steam injection, in-place combustion, etc., but none of them have met with unqualified success. For example, in the case of steam injection procedures, a period of months is often required in order to heat up a sufliciently large body of the oil before the accompanying reduction in viscosity can be exploited. Also, in the now well-known huff-and-puff process for recovering petroleum in which steam is injected into a well for a period of time after which the steam-saturated formation is allowed to soak for an additional interval prior to placing the well on production, much time elapses during which no production is obtained. Also, the relative permeability of the formation to oil decreases owing to the increasing in water saturation. One of the principal drawbacks of the huff-and-puff method, however, is that it does not supply the reservoir with sufficient energy to move the oil toward the producing well in the quantities it should. This process is generally used in reservoirs having little or no energy and hence any oil moving to the producing well depends largely for its energy on that furnished by gravity drainage and by flash vaporization of hot condensate back in the formation when the well is depressurized and placed on production.

Summary of the invention We have now discovered a cyclic method for the recovery of petroleum involving two or more wells in which a suitable [fluid is periodically injected into the United States Patent 3,460,621 Patented Aug. 12, 1969 producing well for the purpose of adding heat in some form to the oil bearing formation. The viscosity of the oil or tar is reduced in this manner and the flow of the petroleum of reduced viscosity to the producing well by gravity drainage is aided by gas injection via an oifset well or preferably a plurality of injection wells distributed about the producing well or wells.

In a preferred embodiment of our invention we employ wells in a conventional five-spot pattern with the producing well in the center and the gas injection wells arranged about the central well. Initially steam is introduced into the formation through the central or producing well and injection is continued at least until the formation within several feet, e.g., 5 or 6 feet, of the well bore, is heated to a temperature of from about 200 F. to 400 F. This may require a steam injection period of from 2 to 5 weeks or even longer. Generally the steam employed is Wet or low grade and may be at a pressure of from 500 to 1500 p.s.i. and at a temperature of from about 400 F. to 600 F.

Alternatively we have found that the reaction rate of oxygen with heavy oil or tars is rapid at temperatures above 200 F. Therefore, that portion of the oil bearing formation penetrated by the producing well need only be heated for a relatively short time by the use of steam or a suitable bottom hole heater to a temperature above 200 F. after which the temperature buildup in the producing well is continued by injection of air.

After the temperature in the producing well has been brought to the proper level, steam or air injection in the producing well is stopped and air is injected into the formation via the surrounding wells. On contact with the air thus injected, the hot petroleum near the producing well begins to burn initiating reverse combustion. Product gases from the process flow toward the producing well tending to push along with them the tar of reduced viscosity. The time schedule for intermittent injection of steam or otherwise heating up the formation around the producing well and effecting reverse combustion depends on the rate of tar production at the steam input well. The practicability of continuing with the cyclic process depends on the proximity of the wells, local conditions and production rates. It is contemplated that we may discontinue the cyclic feature of our invention and continue only with air injection into the formation via the perimeter wells. On continuation of air injection the burning front will reverse its path and proceed in the same direction through the reservoir as the flow of air. Production of tar and product gases continue at the steam input well and the relatively large volume of depleted tar sand around this well is found to be favorable to tar drainage.

Alternatively, air may be introduced into the perimeter wells simultaneously with the steam injection step. On completion of the latter the large reservoir of air thus introduced can be used as additional energy to force the oil of reduced viscosity toward the producing well. Steam injection can then be resumed when production into the central well decreases to a rate below the economic level.

Instead of injecting air at the perimeter wells, we may employ other gases to promote the gas drive, such as a light hydrocarbon mixture, e.g., natural gas, carbon dioxide, flue gas or nitrogen. If any of these gases are used this means that the process will be cyclic with heating periods in the steam input well followed by gas injection in the perimeter wells. If desired, a positive pressure, e.g., 30-40 p.s.i., may be maintained on the reservoir through the perimeter wells during steam injection by continuing the gas injection step.

The process of our invention may be considered cyclic, similar to the huff-and-puif process currently being used in many heavy oil fields in California and elsewhere.

However, in our process, additional energy is supplied by the expanding gas and/or combustion products to drive the hot petroleum to the producing well during the depressuring step or producing cycle. The cycle may be repeated many times and the steam and air rates may be generated over a wide range as indicated by the quantity and rate of heavy oil production. Our invention simply employs the energy of expanding gas to drive heavy oil heated by steam toward the producing well in a system such as described herein. In the case of air, we also have the additional benefit from heat generated by the oxygen reacting with hot tar or heavy oil in the zone around the steam injection well.

Description of the drawings The process of our invention is further illustrated by the accompanying drawings in which- FIGURE 1 is a diagrammatic representation of one embodiment employing a five-spot pattern with the central steam injection or producing well surrounded by air or gas injection wells.

FIGURE 2 represents a segment taken along line 2-2 of the pattern shown in FIGURE 1.

FIGURE 3 is a schematic plot of a five-spot pattern illustrating the different zones formed while steam is injected in the center well and air is simultaneously introduced through the corner wells.

In carrying out an embodiment of our invention, and referring to FIGURES 1 and 2, and steam is injected into central well 4 at a rate of. from about 6,000 to 20,000 lb./ hr. at pressures of from 250 to 2500 p.s.i. Injection of steam as shown by the dashed arrows, into heavy oil bearing formation 6 via perforations 8 is accomplished through tubing 10 which is held in place by means of packer 12 thus defining a confined space 14 communicating with formation 6 through perforation 8. This operation is continued for example from 3 to 4 Weeks, at the end of which time the formation for a distance of three to five feet from well 4 is heated to about 450 F. This heated area contains a portion of the injected steam as vapor. If the well is given time to soak, the steam is condensed into saturated water because of the heat losses primarily due to heat condensation into the surrounding formation.

The low API gravity crude in formation 6 has a viscosity of 2000 cp. at 125 F. but its viscosity is only about 35 cp. at 250 F. At a temperature of 450 F. the viscosity is reduced to 2.5 cp. or a reduction of 800 to 1. The temperature obtainable in producing sands is directly related to injection pressure. At an injection pressure of 450 p.s.i.a., the corresponding steam temperature is 456 F. and the viscosity is 2.5 cp. Bottom hole temperature at termination of the steam injection cycle usually varies between 350 F. and 450 F. Thus at these temperature conditions, oil of reduced viscosity in formation 6 flows by gravity drainage into well 4 and may continue to do so for a period of several months.

Eventually, however, the production rate becomes uneconomical if no additional energy is introduced into the reservoir. As contemplated by our invention, after the steam injection cycle, air is introduced into perimeter wells 16 via tubing 18 and then into formation 6 as shown by the arrows via perforation 20. The air injection rate is about 500,000 c.f./ day and the pressure is approximately 1500 p.s.i. The purpose of this step is at least twofold. The relatively large volumes of air added to the reservoir under these conditions aid the flow of oil to well 4. Additionally, when the oxygen in the air thus introduced contacts oil in the vicinity of well 4 at a temperature of 200-450 F., conditions are provided which favor reverse combustion. This in turn creates additional volumes of hot combustion products which travel toward well 4 tending to force the oil ahead toward well 4. The reverse combustion phase may continue with the burning front approaching well 16, thereafter reversing itself and traveling as a folward combustion front toward well 4 as described in Canadian Patent No.

If the forward combustion phase of the process goes to completion, oil production is terminated. However, in the majority of cases, the reverse burning followed by forward burning, referred to above, does not generally occur without incident and, accordingly, periodic or cyclic additions of heat to the formation via well 4 are required to sustain production, in which case the above steam injection step should be repeated.

A truly cyclic procedure is employed when a gas such as natural gas, carbon dioxide, flue gas or the like is substituted for air. With such gases,- additional energy is taken into the reservoir to assist in forcing the hot oil into central well 4. However, ultimately the oil producing rate falls below an economic level owing to the increase in oil viscosity resulting from heat loss and the steam injection portion of the cycle must be repeated.

In FIGURE 3, air is injected at the four corner wells simultaneously with the steam injection at central well 4. The air injection pressure may be somewhat less than the steam pressure. Zone 5 nearest and around well 4 contains live steam, hot water, and hot oil (or tar). Surrounding the zone is a zone or bank of water 7 (condensed steam). Water zone 7 temperature decreases from hot (steam condensation temperature) on the inside to cold (formation temperature) on the outside. Then, normal temperature oil or tar sand formation and air are in area 9 outside the water zone.

As the steaming operation continues, water zone 7 tends to increase in size and move outward from the well 4, as shown by the solid arrows, because of the lower surrounding air pressure in area 9. During a soaking period, with central well 4 shut in (no steam injection), water zone 7 tends to move inward toward well 4 as shown by the dashed lined arrows.

Upon completion of the steaming (or soaking) part of the cycle, central well 4 is put on production. Hot oil (or tar) is produced from the steam heated zone 5 by a pressure drive generated by the reduced pressure (to atmospheric) at central well 4 and, (1) the back flow of injected steam and steam formed by water flashing in hot zone 5; and (2) the flow of combustion product gases toward the well. The air pressure on the outside of water zone 7 is very important at this time. This pressure drives the water back into steam heated zone 5 where it flashes and drives tar or oil toward well 4. When the air, following the water, reaches hot tar combustion occurs and additional heat is generated. The resulting combustion product gases then flow and drive tar toward the well.

There are several specific advantages to pressuring the formation with air (at corner wells 16) at the same time steam is being injected into central well 4. (1) Since the steam injection time may extend from several days to a few weeks, a relatively long time is available to get the desired air pressure on the formation. Thus, a smaller air injection system could be used then if air injection were started at time of oil production step. Also, the formation permeability may be low making a fast air pressurizing step impossible. (2) Continuous air injection would give a maximum, and more uniform pressure drop between the air zone and producing well for driving tar during production. (3) A higher and uniform air pressure outside the water zone also minimizes Water channeling during the steaming part of cycle.

From the foregoing description it will be appreciated that a number of advantages are afforded by conducting steam injection operations in accordance with our invention. Thus we have taught a flexible process for recovering highly viscous oils as well as oils of intermediate gravity by a combination of steam and gas injection from a multiple well system. When air is injected as the auxiliary energy source and combustion is difiicult to sustain or a non-combustion supporting gas is employed in place of air, a cyclic process is provided by which a high percentage of the oil in place can be recovered. When using steam to heat the area around the producing well, a relatively uniform combustion front can be formed by injection of air via an off-set well.

Although air or gas injection may be discontinued while steam or other means is employed to heat the central well, our invention contemplates a process in which heating of the central well proceeds simultaneously with the air or gas injection step occurring in the perimeter wells. Also while our invention is applicable to reservoirs having formation pressures not in excess of 150 p.s.i., we would like to point out that it is likewise suitable for use in higher pressure reservoirs.

We claim:

1. In a method for the recovery of petroleum from an underground deposit thereof by thermal means, said deposit being penetrated by a producing well and at least one injection well and having suflicient permeability in its native state to permit liquid flow into said producing well by gravity drainage, the improvement which comprises heating that portion of said deposit by means of steam injection at least within from 5 to 6 feet of said producing well until the temperature of said portion is at from about 200 F. to about 400 F. whereby the viscosity of said petroleum in said heated portion is substantially reduced,

forcing the resulting petroleum or reduced viscosity which is in the immediate vicinity of said producing well into the latter by introducing air into said deposit While in its natural state of permeability via said at least one injection well, whereby the oxygei in said air reacts with the hot petroleum present with in at least from 5 to 6 feet of said producing well 11 form additional heat and combustion gases arount said producing well thus facilitating the flow of petro leum into said producing well, and recovering petrole um from said producing well. 2. The method of claim 1 wherein said producing we] is depressured prior to the introduction of said air.

3. The method of claim 2 wherein a plurality of in jection wells are arranged about a central producing well 4. The method of claim 1 wherein the heating and ai injection steps are effected simultaneously.

References Cited UNITED STATES PATENTS 3,064,728 11/ 1962 Gould 166-1 3,259,186 7/1966 Dietz 166-1 3,280,909 10/1966 Closmann et al. 166-11 3,284,281 ll/1966 Thomas 166-11 2,897,894 8/1959 Draper et al. 166-! 2,917,112 12/1959 Trantham. et al. 166-11 3,040,809 6/1962 Pelzer 166-11 3,062,282 11/1962 Schleicher 166-1 STEPHEN J. NOVOSAD, Primary Examiner U.S. Cl. X.R. 166-272, 303 

